1. Field of the Invention
The subject matter of this invention is related to molded case circuit breakers in general and in particular to mechanisms for relieving gas pressure built up within molded case circuit breakers during the contact opening operation.
2. Description of the Prior Art
Molded case circuit breakers are well known. A sample of a molded case circuit breaker may be found in U.S. Pat. 4,503,408 issued Mar. 5, 1985 to Mrenna et al., and assigned to the present assignee. In particular, a molded case circuit breaker generally comprises a two-piece molded casing. There is a top and a bottom part, inside of which is disposed an operating mechanism and separable main contacts.
Generally, the top and bottom portion of the case are either securely butt joined or joined with a locking mechanism, so as to secure the case and to prevent interference with or contact with operating mechanisms, live portions and separable main contacts disposed within the closed casing. A disadvantage, however, is associated with this construction feature, in that gas is generated within the confines of the circuit breaker case during a circuit breaker interruption operation. This gas is usually the result of the electrical arc which is drawn during the contact separation interval which interacts with the molded case material or other material within the circuit breaker. The gas generated must be vented to an external environment. Usually vent apparatus are provided at the rear or load terminal region of the circuit interrupter to achieve this purpose. Such an arrangement can be found in U.S. Pat. No. 4,639,564, entitled "Circuit Breaker with Arc Chamber Vent": issued Jan. 27, 1987 to Grunert et al., and assigned to the present assignee. As can be understood, the maximum current interrupting capability of the circuit breaker is limited by its ability to exhaust undesirable gaseous products. The amount of current which can be safely interrupted is directly related to the heat and size of the electrical arc, which in turned is related to the amount of gas generated. If the gas generation is so high as to exceed the capability of exhausting it, the case itself could be impaired, such as, by destruction, etc. However, in most prior art applications the case is made stronger and larger than it is otherwise necessary to interrupt the expected current in an overload condition to compensate for this.
However, if a way could be found to increase the interrupting capability of the circuit interrupter by providing a alternate or emergency gas exit function, such function would be desirable as the size and/or strength of the case could be reduced.